This invention relates to a development unit for use in an image forming system. Specifically, this invention relates to a development unit including a charge-metering member.
In conventional image forming systems, the image forming process includes charging a photoconductive image member to a substantially uniform potential to sensitize the surface thereof. A charged portion of the photoconductive surface is exposed at an exposure station to a light image of an original document to be reproduced.
Exposing an image of an original document as such at the exposure station records an electrostatic latent image of the original image onto the imaging member. The recorded latent image is subsequently developed using a development subsystem by bringing a charged dry or liquid developer material into contact with the latent image using a developer roll. The toned image is then transferred to paper or another print substrate and permanently fused to the substrate to form a xe2x80x9chardcopyxe2x80x9d of the original image.
A commonly used development technique involves a single component developer material, comprised mainly of toner particles. In a typical single component development system, each toner particle has both magnetic properties (to allow the particles to be magnetically conveyed to the imaging member) and an electrostatic charge (to enable the particles to adhere to the imaging member). In such a system, the developer roll is in the form of a cylindrical sleeve which rotates about a stationary magnet assembly. The magnetized toner particles adhere to the rotating sleeve by the force of the stationary magnets with the sleeve.
In single component development, the particle of toner is charged by a charge-metering member. A charge-metering member is typically in continuous contact with the toner particles along one portion or longitude of the developer roll. The charge-metering member performs two simultaneous functions: it allows a uniform metered layer of toner to pass underneath, and uniformly charges the toner that is mechanically metered. That is, the action of the toner particles rubbing against the charge-metering member and each other while being metered by the charge-metering member induces a charge on the toner particles. The uniformity of the nip formed between the charge-metering member and the developer roll plays a significant role in creating a uniform charge and uniform layer of toner across the developer roll. The charged particles on the surface of the developer roll are advanced towards the imaging member, and then transferred onto the imaging member in image-wise configuration to form a developed toner image on the imaging member. The toner image is subsequently transferred and fused to the paper.
In the prior art, the charge-metering member typically comprises an angled, resilient straight edge blade urged against the surface of the developer roll along the length thereof. The blade consists of a metal substrate with an elastomeric coating, or an elastomeric pad disposed at the free end of the blade. The charge-metering blade is oriented so that the elastomeric portion of the blade contacts toner particles on the surface of the developer roll in order to smooth the layer of toner particles and induce a charge therein.
A significant disadvantage to conventional charge-metering blades is that they deteriorate rather quickly. Particularly, the surface of the blade that contacts the toner particles tends to wear down over time. As the charge-metering member is responsible for creating a uniform layer of toner and a uniform charge of toner across the developer roll, a deteriorated or worn charge-metering blade compromises print quality. A smooth contact surface on the charge-metering member is necessary to promote an even layer of toner and an even charge of the toner particles. When a charge-metering member wears out, indicated by degradation in the quality of the final image, it is necessary for a customer to replace it with a new charge-metering member. Often, this involves replacing a number of system elements that are collectively provided in a Customer Replaceable Unit (CRU). When a charge-metering member wears out, the entire CRU must be replaced, which is an expensive and time-consuming process. In a typical single component development system, the charge-metering blade deteriorates and is replaced after between about 12,000 and 15,000 copies.
The present invention provides a single-component development unit having a charge-metering member and a motion-imparting system in cooperation with the charge-metering member for periodically shifting the charge-metering member to expose a fresh surface for charging and metering toner on a developer member. After a certain print interval, the motion-imparting system automatically replaces a worn charge-metering surface with a new and unworn surface by shifting the charge-metering member. The present invention extends the life of the charge-metering member and the development unit as a whole. In addition, the present invention ensures that a smooth surface is continually used to charge and meter the toner. Consequently, the quality of a final image is significantly improved for an image forming process that utilizes the teachings of the present invention.
According to one aspect of the present invention, the developer unit comprises a housing, a developer member, and a rotatable charge-metering member that cooperates with the developer member to charge said toner particles on the developer member and to regulate a thickness of a layer of toner particles on the developer member.
According to another aspect, the charge-metering member is a cylindrical roll, and a motion-imparting system is designed to rotate the cylindrical roll after a predetermined time period to expose a fresh charge-metering surface.
According to another aspect, the charge-metering member comprises an endless belt, and a motion-imparting system is designed to advance the belt after a predetermined time period to expose a fresh charge-metering surface.
According to still another aspect, the charge-metering member comprises a rectangular blade, and a motion-imparting system is designed to shift the rectangular member to present a fresh charge-metering surface.